Another blower question

[635911]

Hi All,

I have an RHD 1986 635 in pretty good shape.

Yesterday fuse 20 blew. Put in another, immediately blew.

Today, researched the forum/Bentley/ETM etc, pulled the heater blower out, a/c off, fan rotary controller to off, tried another fuse, pop! Blower was disconnected, but free spinning anyway.

Pulled the blower switching relay and swapped it out. Everything off. Pop!

11.6 volts across the fuse at all times when ignition on.

I am guessing a harness short somewhere, but really don't know where to start with finding it.

Any tips on tracking a short, or what else this could are greatfully received.

Toby

[daz635]

Hi Toby

Looking at the ETM for fuse 20 the items that I can see in the circuit are, the blower fan speed controller, ac selector switch, ac and heater fans with their associated NPN transistors and the blower select relay.

From what I know the Blower fan rotary switch controls the power flow to all other parts of the system and is the first switch in the system after fuse 20. Try disconnecting the blower fan rotary switch. If it still blows you will know the short is between this switch and fuse 20. If not you will know the issue is the switch or further down.

I cannot see any other systems connected to fuse 20 from the ETM

[jacko]

If you have an old BMW, you should sleep with the ETM under your pillow. Seriously, it's a great tool and free!

[Brucey]

word of warning; the available ETMs are for North American models. RHD ones are different to that and the fuse allocations do vary even within RHD models. Amazingly some RHD models were even supplied with owner's handbooks that don't give the correct fuse allocations, i.e. the list inside the handbook is incorrect.

So my suggestion is to check other sources apart from the ETMs (including the owner's handbook) for fuse allocations. Also worth checking other third-party sources such as the Haynes manual. For '85, 86, 87 chrome bumper RHD cars I have found the Haynes manual (for 3 and 5 series) to be the most accurate source in many cases.

If you know the exact model (four digit model code) and build date maybe someone else with a similar car can check what stops working when you pull fuse 20.

cheers

[jacko]

Brucie, I didn't realize you "wrong siders" didn't get decent ETMs. Ours over here are very good!

The price you pay I guess.

[635911]

HI All,

Thanks for the helpful replys.

I did mention that I had consulted the ETM, but found what Brucey now confirms - they are not universal.

On my car (June '86 build, Australian delivered, chrome bumper 635) it is fuse 20 that pops. Nothing seems to go out apart from the two fans.

Jacko, thanks for the info. I intend to follow your advice and pull the switch out and check for continuity/short from there first.

Any other tips guys?

Thanks,

Toby

[Brucey]

IIRC there is a permanent live connection to the large MOSFET transistor that (is adjacent to and) drives the blower fan. If (when) this fails, one possible failure mode is a dead short to ground.

IIRC (do check this, it has been a while) the heat sink for the transistor is grounded and there is a thin insulating shim between the MOSFET and the heat sink. If this fails (or the motor itself fails to a dead short) then the relevant fuse will blow immediately.

If you disconnect the wires from the MOSFET, it will eliminate this as a potential path to ground.

The same logic applies to the AC fan too, I think. Failing these parts being bad, there are not so many parts of this circuit that will take fuse-blowing currents without themselves suffering damage. This means that you would be looking for a failure in the insulation on the wiring somewhere or a catastrophic fault in (say) the fan speed control unit (which also has a ground connection).

cheers

[635911]

Thanks again!

I have dug out the blower speed unit, disconnected it and pop! I see this as good news. Hard to get bits might be OK!

Brucey, thanks so much for the next step. My ETM indeed says power goes to the transistors, so next step is back into heater blower housing and pull the three transistor wires (two blower wires) and see what I get.

Thanks again to all, I am beginning to enjoy solving problems on this car, knowing there is such a supportive and knowledgable crew out there.

Stay tuned........

Toby

[635911]

Hi Guys,

I removed the heater blower motor and fan, undid the three wires to the heater blower transistor, tested with a new fuse... POP!

I then unplugged the large multi connector that is fitted to the side of the temperature know housing. Retested. NO POP!

So, I think this means the short is definately "downstream" of the connector. Is that correct?

Any suggestions on working with this connector (ie pins are exposed) to test for continuity and/or voltage (as across the fuse terminals to "see" the short?

I guess I should not see any voltage except on the pin that is shorted (ie red multimeter lead in "upstream" side black lead in "downstream" side, when fan rotary switch is plugged in, but switched to off?

Any other thoughts on tackling the next step?

Thanks,

Toby

[baders]

Toby have you checked the A/C blower unit the same way you have checked the heater blower ? The weak point in these systems is always the transistors.

[635911]

Hi Baders,

I just tried to delete the previous post as I realise the process would be of no use, so I will need to dive into the A/C department and disconnect fan, restest, disconnect transistor, retest.

If that fails, I'll bring Brucey over for a holiday.....

Thanks for the tip,

Toby

[635911]

A little progress.

Inspired by Baders, I went into the gloom under the steering wheel, and (using my trusty companion (The ETM)) I identified the A/C Blower connector. I pulled it out and retested with a new fuse. NO POP!

Connected Heater Blower transitor and fan. Test. No POP! Heater fan operational! YAY!! 50% functionality (and it's getting cool here now, so heater is better than A/C).

So, suspects are the A/C Blower Transistor and A/C Blower fan motor. I had the A/C evaporator etc out a few months ago and lubed the motor spindle bearings, so hoping it's the transistor.

I will need a bit more time and space to rip the console out, so that will be Saturday.

Tips on testing from the back of the connector?

I might buy a transistor just in case. Brucey (et al), any view on 2N5302G versus 2N3055 versus 2N3773??

Thanks again guys,

Toby
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